Brian’s blog entry on A thought on mocking filesystems shows that he understands the point.
One of the potential risks with mock-based testing is of simply reproducing the implementation of the Code Under Test, but that only really gets in the way if you can’t change the collaborators you’re mocking. The point of the exercise is to discover the interfaces of collaborating objects from the point of the view of the client, the CUT. This leads to a coding style with small, focussed interfaces which might be implemented by larger classes. Each object gets just what it needs from its neighbours.